I think recoil is an acquired taste. The more shooting you do the less recoil is likely to bother you - right up until the gun hurts you that is. When that happens it can be difficult for your brain to get past the association of "what you are about to do is going to hurt you". This can be brought on in ways other than by the recoil we feel on the shoulder. You could get tagged by the scope on a hard kicking rifle, the front swivel could cut your hand, or perhaps the knuckle of your shooting hand gets wrapped by the rear of the trigger guard...all of these things can lead to a flinch. The only flinch cure I am aware of is to start shooting lesser rifles and work your way back to the big stuff.

I think most people can be trained to shoot a properly stocked .30-06. If they are unable to do this they will probably never progress to the point that they could shoot a .375 well, never mind anything bigger. The new shooter simply must accept that these things are designed to be used by normal people. I remember a fellow who wanted to shoot my .416 Rigby, and I assumed - incorrectly that he had heavy rifle experience. I was just about to yell STOP when he fired. My handloads pushed a 350 gr. X over 2800 fps, and as a result the rifle had a little push to it. When he fired I could see daylight between the recoil pad and his shoulder. He has never wanted to shoot a powerful rifle since.

As to stock fit - it is absolutely critical. The most painful rifle I ever shot was a little Mosin Nagant carbine in 7.62X54R. If I had not had experience with other rifles prior to shooting that thing, I would of just gone back to shooting a 6.5 Swede, believing that shooting powerful rifles was beyond me.